<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let year = +prompt('请输入年份')
        if ((year % 4 == 0 && year % 100 !== 0) || year % 400 == 0) {
            alert('是闰年')
        }
        else {
            alert('不是闰年')
        }
    </script>
</body>

</html>